EMI gasket

ABSTRACT

The invention relates to an EMI gasket formed by joining coated steel ( 3 ) and a contact backing ( 1 ). The coated steel ( 3 ) has elongated openings ( 4 ), into which projections ( 2 ) provided on the contact backing fit, seizing by their edges the edges of the openings.

This invention relates to an EMI gasket formed by joining coated steel and a contact s backing.

The EMI gasket of the invention is intended for application to electronic casings and racks to ensure the EMC requirements posed on the finished apparatus. The use of the gasket ensures a completely sealed casing (cf. a Faraday cage) at critical points (of lateral walls).

There are prior art solutions in which an EMI gasket is dimensioned such that contacts directly an electrically conductive surface of precoated sheet steel (cf. an STU casing).

A solution has also been manufactured and tested that comprises an even contact backing made of spring steel for the EMI gasket. This spring steel has made electric contact with the electrically conductive surface of precoated sheet steel. Shield attenuation measurements have indicated attenuation results of only about 30 dB at a frequency of 2,000 MHz.

However, the electrically conductive coating mentioned above, which has been provided on precoated sheet steel plates, has proved susceptible to mechanical damage, and in the practice, the electric coating has not ensured sufficiently low resistance between the EMI gasket and the sheet steel.

This invention has the objective of eliminating the drawbacks mentioned above. The EMI gasket of the invention is characterised by the coated steel comprising elongated openings, in which projections from the contact backing are inserted, seizing by their edges the edges of the openings. Different embodiments of the invention are described in The dependent claims of the set of claims.

The EMI gasket of the invention serves to improve electric contact between a precoated steel cage and rack and a second EMI gasket by the backing surface (contact backing) of the metal EMI gasket and attachment of the contact backing to the precoated sheet steel. The fixed contact backing provides an even surface with the desired length, on which the EMI gasket can be either mounted directly in an electrically conductive manner, or against which mechanical and electric contact can be made by a second EMI gasket

The use of an EMI gasket provides conditions for long-term and reliable electric contact, such that is required when forming an interface shielding against electric interference from an electronic casing or rack

A protective interface is needed either to protect electronic equipment from external interfering radiation at radio frequency or to protect the environment from intrinsic radiation generated in the electronic equipment.

The differences in the electric potentials of the selected materials are small, given the same basic material of the two members and the very close potentials of zinc coating and iron. This contributes to a minimised risk of galvanic corrosion.

The invention is explained by means of an example and with reference to the accompanying drawings, in which

FIG. 1 shows the members of an EMI gasket separated from each other,

FIG. 2 shows the same members as FIG. 1, but joined to form the EMI gasket, and

FIG. 3 shows an enlargement of the location marked with a dotted line in FIG. 2, viewed from the top and partially cut.

A contact backing 1 of the desired length is made from stainless sheet steel. Projections 2 are provided on both sides of the contact backing. The finished contact backing is bent at angles.

The contact backing 1 is mounted in advance in a backing groove 3 made of coated steel plate, the other edge of the groove not being bent at this stage. At the bottom of the groove, at both edges, rectangular openings 4 are provided with the same dimensioning as that of the projections 2 on the contact backing. The length of the openings 4 matches the projection 2 of the contact backing with high precision, for the projection to cut material from the precoated sheet steel edge 3 as the members are joined. 

1. An EMI gasket consisting of a joint of coated steel (3) and a contact backing (1), characterized in that the coated steel (3) comprises elongated openings (4), projections (2) on the contact backing (1) fitting into the openings and seizing by their edges the edges of the openings.
 2. An EMI gasket as defined in claim 1, characterized in that coated steel (3) is an U-shaped groove, having at its bottom pairs of openings (4), into which projections (2) on the contact backing (1) fit as the members are joined.
 3. An EMI gasket as defined in claim 1 or 2, characterized in that the contact backing (1) is a U-shaped groove turned upside down, from which projection (2) protrude into opposite directions.
 4. An EMI gasket as defined in claim 3, characterized in that the projection (2) has a shape tapering conically outwardly from its edges so that the edges seize the edges of the openings (4).
 5. An EMI gasket as defined in claim 4, characterized in that the coated steel groove (3) is made of zinc-coated iron and the contact backing (1) is made of stainless sheet steel. 